Convert date-time faithfully in QDateTimeEdit::setDateTime()

Previously, setDateTime() was documented to ignore the new date-time's
time-spec. It used the date and time (determined using that timespec)
with the QDateTimeEdit's configured spec. It is debatable whether that
really counts as ignoring its time-spec. All the same, that's what it
did. Fixing it is a behavior change. Added tests.

[ChangeLog][QtWidgets][QDateTimeEdit] QDateTimeEdit::setDateTime() now
converts the new datetime to the QDateTimeEdit's time-spec, rather
than combining its date and time (determined using the time spec it
came with) with the QDateTimeEdit's date and time.

Fixes: QTBUG-71181
Change-Id: Ibf0bd87723c3957ca00a2199d51d992032ef57ee
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Konstantin Shegunov <kshegunov@gmail.com>

src/widgets/widgets/qdatetimeedit.cpp

@@ -53,6 +53,9 @@
 #include <qlayout.h>
 #include <qset.h>
 #include <qstyle.h>
+#if QT_CONFIG(timezone)
+#include <QTimeZone>
+#endif
 
 #include <algorithm>
 
@@ -218,8 +221,8 @@ QDateTimeEdit::~QDateTimeEdit()
   \property QDateTimeEdit::dateTime
   \brief the QDateTime that is set in the QDateTimeEdit
 
-  When setting this property the timespec of the QDateTimeEdit remains the same
-  and the timespec of the new QDateTime is ignored.
+  When setting this property, the new QDateTime is converted to the timespec of
+  the QDateTimeEdit, which thus remains unchanged.
 
   By default, this property is set to the start of 2000 CE. It can only be set
   to a valid QDateTime value. If any operation causes this property to have an
@@ -243,11 +246,14 @@ void QDateTimeEdit::setDateTime(const QDateTime &datetime)
 {
     Q_D(QDateTimeEdit);
     if (datetime.isValid()) {
+        QDateTime when = d->convertTimeSpec(datetime);
+        Q_ASSERT(when.timeSpec() == d->spec);
+
         d->clearCache();
-        const QDate date = datetime.date();
+        const QDate date = when.date();
         if (!(d->sections & DateSections_Mask))
             setDateRange(date, date);
-        d->setValue(QDateTime(date, datetime.time(), d->spec), EmitIfChanged);
+        d->setValue(when, EmitIfChanged);
     }
 }
 
@@ -1706,6 +1712,25 @@ QDateTimeEditPrivate::QDateTimeEditPrivate()
 #endif
 }
 
+QDateTime QDateTimeEditPrivate::convertTimeSpec(const QDateTime &datetime)
+{
+    Q_ASSERT(value.toDateTime().timeSpec() == spec);
+    switch (spec) {
+    case Qt::UTC:
+        return datetime.toUTC();
+    case Qt::LocalTime:
+        return datetime.toLocalTime();
+    case Qt::OffsetFromUTC:
+        return datetime.toOffsetFromUtc(value.toDateTime().offsetFromUtc());
+#if QT_CONFIG(timezone)
+    case Qt::TimeZone:
+        return datetime.toTimeZone(value.toDateTime().timeZone());
+#endif
+    }
+    Q_UNREACHABLE();
+}
+
+// FIXME: architecturaly incompatible with OffsetFromUTC or TimeZone as spec (QTBUG-80417).
 void QDateTimeEditPrivate::updateTimeSpec()
 {
     minimum = minimum.toDateTime().toTimeSpec(spec);

src/widgets/widgets/qdatetimeedit_p.h

@@ -109,6 +109,7 @@ public:
 
     void updateCache(const QVariant &val, const QString &str) const;
 
+    QDateTime convertTimeSpec(const QDateTime &datetime);
     void updateTimeSpec();
     QString valueToText(const QVariant &var) const { return textFromValue(var); }
 

tests/auto/widgets/widgets/qdatetimeedit/tst_qdatetimeedit.cpp

@@ -262,6 +262,8 @@ private slots:
     void timeSpec();
     void timeSpecBug();
     void timeSpecInit();
+    void setDateTime_data();
+    void setDateTime();
 
     void monthEdgeCase();
     void setLocale();
@@ -3476,6 +3478,54 @@ void tst_QDateTimeEdit::timeSpecInit()
     QCOMPARE(widget.dateTime(), utc);
 }
 
+void tst_QDateTimeEdit::setDateTime_data()
+{
+    QTest::addColumn<Qt::TimeSpec>("spec");
+    QDateTime localNoon(QDate(2019, 12, 24), QTime(12, 0), Qt::LocalTime);
+#if 0 // Not yet supported
+    QTest::addColumn<int>("offset");
+    QTest::addColumn<QByteArray>("zoneName");
+
+    QTest::newRow("OffsetFromUTC/LocalTime")
+        << Qt::OffsetFromUTC << 7200 << ""
+        << localNoon << localNoon.toOffsetFromUtc(7200);
+#if QT_CONFIG(timezone)
+    QTest::newRow("TimeZone/LocalTime")
+        << Qt::TimeZone << 0 << "Europe/Berlin"
+        << localNoon << localNoon.toTimeZone(QTimeZone("Europe/Berlin"));
+#endif
+#endif // unsupported
+    QTest::addColumn<QDateTime>("store");
+    QTest::addColumn<QDateTime>("expect");
+    QTest::newRow("LocalTime/LocalTime")
+        << Qt::LocalTime // << 0 << ""
+        << localNoon << localNoon;
+    QTest::newRow("LocalTime/UTC")
+        << Qt::LocalTime // << 0 << ""
+        << localNoon.toUTC() << localNoon;
+    QTest::newRow("UTC/LocalTime")
+        << Qt::UTC // << 0 << ""
+        << localNoon << localNoon.toUTC();
+    QTest::newRow("UTC/UTC")
+        << Qt::UTC // << 0 << ""
+        << localNoon.toUTC() << localNoon.toUTC();
+}
+
+void tst_QDateTimeEdit::setDateTime()
+{
+    QFETCH(const Qt::TimeSpec, spec);
+#if 0 // Not yet supported
+    QFETCH(const int, offset);
+    QFETCH(const QByteArray, zoneName);
+#endif // configuring the spec, when OffsetFromUTC or TimeZone
+    QFETCH(const QDateTime, store);
+    QFETCH(const QDateTime, expect);
+    QDateTimeEdit editor;
+    editor.setTimeSpec(spec);
+    editor.setDateTime(store);
+    QCOMPARE(editor.dateTime(), expect);
+}
+
 void tst_QDateTimeEdit::cachedDayTest()
 {
     testWidget->setDisplayFormat("MM/dd");